Layout is one of the most basic skills of PCB design engineers. The quality of the routing will directly affect the performance of the whole system. Most of the high-speed design theories must be realized and verified by layout. Therefore, wiring is very important in the high-speed PCB design. In the following, the rationality of the actual wiring will be analyzed according to some possible situations, and some optimized routing strategies are given. This paper mainly discusses the right angle line, differential line, snake line and so on. 1. right angle routing is generally a situation that PCB wiring should be avoided as much as possible, and it is almost one of the standards to measure the quality of wiring. So how much influence will right angle routing have on signal transmission? In principle, the right angle line will change the line width of transmission line, and cause the discontinuity of impedance. In fact, not only right angle, corner, sharp angle can cause impedance change.
The influence of right angle routing on signal is mainly reflected in three aspects: one is that corner can be equivalent to the capacity load of transmission line, which can slow up time; Second, the discontinuous impedance will cause the reflection of the signal; Third, EMI produced by the right angle tip.
The parasitic capacitance caused by the right angle of transmission line can be calculated by the following empirical formula: C = 61W (ER) 1/2z0, in the above formula, C refers to the equivalent capacitance of the corner (unit: PF), and W refers to the width of the line (in inches), ε R refers to the dielectric constant of the medium, Z0 is the characteristic impedance of the transmission line. For example, for a 4mils 50 ohm transmission line（ ε R is 4.3), the electric capacity of a right angle is about 0.0101pf, and the change of rise time caused by it can be estimated: t10-90% = 2.2 * c * Z0 / 2 = 2.2 * 0.0101 * 50 / 2 = 0.556ps. The capacitance effect caused by right angle routing is extremely small.
Because the line width of the right angle line increases, the impedance will decrease, and a certain signal reflection phenomenon will occur. We can calculate the equivalent impedance after the increase of the line width according to the impedance calculation formula mentioned in the transmission line section, and then calculate the reflection coefficient according to the empirical formula: ρ=( The impedance change caused by the general right angle line is between 7% and 20%, so the maximum reflection coefficient is about 0.1. Moreover, from the following figure, it can be seen that the impedance of transmission line changes to the minimum in a long time of w/2 line, and then returns to normal impedance after w/2 time. The whole time of impedance change is very short, often within 10ps. Such rapid and small changes can be ignored for general signal transmission.
Many people have such understanding of right angle line, and think that the tip is easy to emit or receive electromagnetic waves and generate EMI, which is one of the reasons many people think cannot go right angle. However, many practical tests show that the right angle line does not produce a significant EMI than a straight line. The accuracy of the test is restricted by the performance and testing level of the instrument, but at least one problem is explained. The radiation of the right angle line is less than the measurement error of the instrument itself.
In general, right angle is not as terrible as it is supposed to be. At least in applications below GHz, any effects such as capacitance, reflection, EMI produced by them can hardly be reflected in TDR test. The emphasis of high-speed PCB design engineers should be on layout, power / ground design, routing design, hole passing and other aspects. Of course, although the influence of right angle routing is not very serious, it is not that we can take right angle line in the future. Attention to details is the essential basic quality of every excellent engineer. Moreover, with the rapid development of digital circuit, the signal frequency processed by PCB engineers will also be improved continuously to RF design field above 10GHz, These small right angles can be the focus of high-speed problems.
2. differential signal is widely used in high-speed circuit design. The most important signal in the circuit is often designed by differential structure. What is the other thing that it is so popular? How can PCB design ensure its good performance? With these two issues, we will discuss the next part.
What is differential signal? Generally speaking, the driver sends two equal and reverse signals. The receiving end judges whether the logic state “0” or “1” is “1” by comparing the difference between the two voltages. The pair of lines carrying differential signals are called differential lines.
The most obvious advantages of differential signal compared with the common single end signal routing are as follows: a. the anti-interference ability is strong, because the coupling between the two differential lines is very good. When there is noise interference, it is almost coupled to two lines at the same time, while the receiving end is concerned about the difference between the two signals, so the common mode noise can be completely offset. b. In the same way, because of the opposite polarity of the two signals, the electromagnetic fields radiated by them can be offset each other. The closer the coupling is, the less electromagnetic energy is released to the outside world. c. The timing positioning is accurate. Because the switch change of differential signal is located at the intersection of two signals, unlike the ordinary single terminal signal depends on the high and low threshold voltage, it is less affected by technology and temperature, which can reduce the error in time sequence, and is more suitable for circuits with low amplitude signal. The popular LVDS (low voltage differential signaling) is the small amplitude differential signal technology.
For PCB engineers, the most important thing is how to ensure that the difference routing can be fully used in the actual routing. Perhaps as long as anyone who has contacted layout will understand the general requirements of differential routing, that is, “equal length, equal distance”. The equal length is to ensure that the two differential signals keep the opposite polarity and reduce the common mode components; The main purpose of the isometric method is to ensure the difference impedance is consistent and reduce reflection“ The principle of “try to be close to the principle” is also one of the requirements of differential routing. But all these rules are not used to carry the hardware, many engineers seem to have no idea of the nature of high-speed differential signal transmission. The following focuses on several common mistakes in the design of PCB differential signal.
Mistake one: it is considered that the differential signal does not need the ground plane as the return path, or that the differential routing line provides the other party with the return path. The reason for this error is that it is confused by surface phenomena or the mechanism of high-speed signal transmission is not deep enough. From the structure of the receiving end of Fig. 1-8-15, it can be seen that the emitter current of transistors Q3 and Q4 is equal and reverse. The current at the ground is just offset (i1=0), so the differential circuit is insensitive to similar ground bombs and other noise signals that may exist on the power supply and ground plane. Partial backflow cancellation of ground plane does not mean that the differential circuit does not take the reference plane as the signal return path. In fact, in the signal return analysis, the mechanism of differential routing is consistent with that of the common single end routing. That is, the high frequency signal always returns along the circuit with the smallest inductance. The biggest difference is that the differential line is coupled to the ground, There are also mutual coupling. Which one is strong will become the main return path. Figure 1-8-16 is the distribution diagram of geomagnetic field of single terminal signal and differential signal. In PCB circuit design, the coupling between differential lines is small, which usually accounts for only 10-20% of coupling degree, and more of them are ground coupling. Therefore, the main return path of differential circuit still exists in the ground plane. When the local plane is discontinuous, the coupling between the differential lines will provide the main return path in the area without reference plane, as shown in figure 1-8-17. Although the influence of discontinuity of reference plane on differential routing is not serious to the common single end routing, it will reduce the quality of differential signal and increase EMI, so as to avoid as much as possible. Some designers also believe that the reference plane below the differential routing line can be removed to suppress some common mode signals in differential transmission, but it is not advisable theoretically. How to control the impedance? It is inevitable that EMI radiation will be caused if the ground impedance loop is not provided to common mode signal. This method has more disadvantages than advantages.
Mistake two: it is more important to keep equal distance than match line length. In the actual PCB wiring, it is often unable to meet the requirements of differential design at the same time. Due to the distribution of pins, through holes, and the space of the line, the purpose of line length matching must be achieved by proper winding. But the result is that some areas of the difference pair cannot be parallel. How should we choose? Before we conclude, we look at the following simulation results. From the simulation results above, the wave forms of scheme 1 and scheme 2 are almost coincident, that is, the influence caused by the unequal spacing is very small. Compared with other words, the influence of the mismatch of line length on timing is much greater (scheme 3). From the theoretical analysis, the difference between the two pairs is not significant, so the range of impedance change is very small, usually within 10%, which is equivalent to the reflection caused by one through hole, which will not have a significant impact on signal transmission. Once the line length does not match, in addition to the time sequence offset, the common mode component is introduced into the differential signal, which reduces the quality of the signal and increases EMI.
It can be said that the most important rule in the design of PCB differential routing is the length of matching line. Other rules can be flexibly processed according to the design requirements and practical application.
Mistake 3: think that the difference line must depend on very close. It is necessary to make the differential line close to enhance their coupling, not only improve the immunity to noise, but also make full use of the opposite polarity of magnetic field to offset the electromagnetic interference to the outside world. Although this is very beneficial in most cases, it is not absolute. If they can be adequately shielded and free from external interference, we will no longer need to let each other achieve the purpose of anti-interference and EMI suppression through strong coupling. How can differential wiring be isolated and shielded? Increasing the distance between the lines of other signals is one of the most basic ways. The electromagnetic field energy decreases with the square relation of distance. When the line spacing is more than 4 times the line width, the interference between them is very weak, which can be ignored basically. In addition, the isolation of ground plane can also play a good shielding role. This structure is often used in the design of high frequency (10g or above) IC packaging PCB, which is called CPW structure, which can ensure strict differential impedance control (2z0), as shown in figure 1-8-19. Differential routing can also be used in different signal layers, but it is not recommended to use this method because the difference of impedance and hole in different layers will destroy the effect of differential mode transmission and introduce common mode noise. In addition, if the coupling between adjacent two layers is not close enough, the ability of differential line to resist noise will be reduced, but crosstalk is not a problem if the proper distance between the two layers and the surrounding routing can be maintained. EMI is not a serious problem at general frequency (GHz). Experiments show that the radiation energy attenuation of 500mils is 60dB away from 3m, which is enough to meet the electromagnetic radiation standard of FCC. Therefore, designers should not worry about the electromagnetic incompatibility problem caused by the insufficient coupling of differential lines.
3. snake line is a kind of routing method used frequently in layout. The main purpose of this paper is to adjust the delay and meet the requirements of system timing design. Designers should first have such understanding: snake line will destroy signal quality, change transmission delay, and avoid using it as much as possible. But in the actual design, in order to ensure that the signal has enough time to keep, or to reduce the time offset between the same group of signals, it is often necessary to intentionally winding. So what is the effect of snake line on signal transmission? What should we pay attention to when walking? The two most critical parameters are the parallel coupling length (LP) and coupling distance (s), as shown in figure 1-8-21. It is obvious that when the signals are transmitted on the snake shaped route, the coupling will occur between the parallel segments, which is in the form of differential mode. The smaller s is, the larger LP is, the greater the coupling degree will be. The transmission delay may be reduced and the quality of the signal will be greatly reduced due to crosstalk. The mechanism can be referred to the analysis of common mode and differential mode crosstalk in Chapter 3.
The following are some suggestions for layout engineer to deal with snake line: 1. increase the distance (s) of parallel line as far as possible, at least more than 3h, H refers to the distance from signal line to reference plane. Generally speaking, the line around a large bend can almost completely avoid coupling effect as long as s is large enough. 2. reduce coupling length LP, when the double LP delay approaches or exceeds the signal rise time, the crosstalk will reach saturation. 3. the signal transmission delay caused by the serpentine of strip line or embedded micro strip is less than that of micro strip. In theory, the bandline does not affect the transmission rate because of the differential mode crosstalk. 4. for signal lines with high speed and strict requirements on timing, do not follow snake line as much as possible, especially in a small range. 5. snake shaped routing with any angle can be used frequently, as shown in figure 1-8-20, C structure, which can effectively reduce coupling between them. 6. in the design of high-speed PCB, snake line has no so-called filtering or anti-interference ability, and it can only reduce signal quality, so it is only used for timing matching without other purposes. 7. sometimes the spiral route can be considered for winding. The simulation shows that the effect is better than that of normal snake.
